1. Field of the Invention
The present invention generally relates to liquid-crystal display devices, and, more particularly, to a liquid-crystal display device of an active-matrix type having a thin-film transistor and a method of producing such a liquid-crystal display device.
2. Description of the Related Art
Liquid-crystal display devices are compact and consume lower power, and, for these reasons, have been widely used in portable information processing devices, such as notebook-type personal computers. However, the use of liquid-crystal display devices is not limited to portable information processing devices. Actually, the liquid-crystal display devices have started replacing conventional CRT display devices in desktop information processing devices. Moreover, the liquid-crystal display devices are greatly expected to serve as displays for high-definition television (HDTV), and particularly for projection HDTV.
With those high-performance large-area liquid-crystal display devices, the conventional simple matrix driving system is not adequate to satisfy various conditions such as the response rate, the contrast ratio, the color purity, and so forth. Therefore, the active-matrix driving system, in which each pixel is driven by a corresponding thin-film transistor (TFT), is employed. In a liquid-crystal display device, an amorphous silicon liquid-crystal display using amorphous silicon in the active regions of TFTs has been conventionally used. However, the electron mobility of amorphous silicon is small, and cannot satisfy the conditions required in the above high-performance liquid-crystal display devices. Accordingly, it is preferable to use polysilicon TFTs in those high-performance liquid-crystal display devices.
FIG. 1 is a schematic view showing the structure of a conventional active-matrix liquid-crystal display device. As shown in FIG. 1, the liquid-crystal display device comprises a TFT glass substrate 1A that carries a large number of TFTs and transparent pixel electrodes in cooperation with the TFTs, and a counter glass substrate 1B formed on the TFT glass substrate 1A. Between the TFT glass substrate 1A and the counter glass substrate 1B, a liquid-crystal layer 1 is sealed by a sealing member 1C. In the liquid-crystal display device, the transparent pixel electrodes are selectively driven via each corresponding TFT, so that the orientations of liquid-crystal molecules can be selectively varied with each selected transparent pixel electrode in the liquid-crystal layer. Outside the glass substrates 1A and 1B, polarizing plates (not shown) are arranged in a crossed-Nicol state. Inside the glass substrates 1A and 1B, a molecular orientation film (not shown) is formed in contact with the liquid-crystal layer 1, thereby restricting the orientations of the liquid-crystal molecules.
FIG. 2 is a sectional view of the liquid-crystal display device shown in FIG. 1.
As shown in FIG. 2, a large number of pixel TFTs 11 and a peripheral circuit 1PR for driving the pixel TFTs 11 are formed on the TFT glass substrate 1A. Also, connection terminals and a pad electrode 1c are formed outside the sealing member 1C. The peripheral circuit 1PR is also constituted by TFTS, and an interlayer insulating film 1AI is formed in the region enclosed by the sealing member 1C on the TFT glass substrate 1A in such a manner that the interlayer insulating film 1AI covers the peripheral circuit 1PR and the pixel TFTs 11. On the interlayer insulating film 1AI, a large number of pixel electrodes 14 are formed in contact with the respective pixel TFTs 11. On the interlayer insulating film 1AI, a molecular orientation film 1MO is further formed in such a manner that the molecular orientation film 1MO covers the pixel electrodes 14 and is brought into contact with the enclosed liquid-crystal layer 1.
A large number of color filter patterns 1CF corresponding to the pixel electrodes 14 are formed on the glass substrate 1B, and light blocking patterns 1BM are formed between the color filter patterns 1CF. On the counter glass substrate 1B, a flattening insulating film 1BI is formed so as to cover the color filter patterns 1CF and the light blocking patterns 1BM. On the flattening insulating film 1BI, a counter transparent electrode 1ITO is uniformly formed. The counter transparent electrode 1ITO is covered with another molecular orientation film 1MO, which is in contact with the liquid-crystal layer 1. The molecular orientation films 1MO restrict the orientations of the liquid-crystal molecules in the liquid-crystal layer 1.
Furthermore, a first polarizing film 1PL is formed on the lower surface of the TFT glass substrate 1A, while a second polarizing film 1AL is formed on the upper surface of the counter glass substrate 1B, in such a manner that the polarizing axes are perpendicular to each other.
FIG. 3 is an enlarged view of a part of the TFT glass substrate 1A shown in FIG. 1.
As shown in FIG. 3, a large number of pad electrodes 13A that receive scanning signals, a large number of scanning electrodes 13 extending from the pad electrodes 13A, a large number of pad electrodes 12A that receive video signals, and a large number of signal electrodes 12 extending from the pad electrodes 12A, are formed on the glass substrate 1A in such a manner that the extending direction of the scanning electrodes 13 is substantially perpendicular to the extending direction of the signal electrodes 12. At each intersection of the scanning electrodes 13 and the signal electrodes 12, the TFT 11 is formed. Furthermore, the transparent pixel electrodes described before are formed so as to correspond to the respective TFTs 11. Each of the TFTs 11 is selected in accordance with the scanning signal on each corresponding scanning electrode 13, and each cooperative transparent pixel electrode 14 is driven in accordance with the video signal on each corresponding signal electrode 12. In FIG. 3, the pad electrodes 12A and 13A are equivalent to the pad electrode 1c shown in FIG. 2.
On such an insulating glass substrate, however, static electricity is often generated due to various factors during the production of the TFTs. For instance, in a case where the insulating glass substrate is attached to or removed from a processing machine, a transportation means, a jig, or a substrate holder, static electricity enters the substrate from the outside. Also, various plasma processes used for forming the TFTs on the substrate, such as the plasma CVD method, the sputtering method, or the RIE process, might result in the accumulation of static electricity inside the substrate. In these plasma processes, the conductive patterns or the diffusion regions function as antennas, and the differences in effective area among the antennas induce potential differences in the substrate. Since the substrate itself is an insulator, the induced potential differences cannot be cancelled, resulting in unrecoverable permanent damage, partially recoverable semi-permanent damage, overruns due to variations of the threshold voltage, characteristic deterioration due to a decrease of mobility, poor long-term reliability due to potential problems, or the like. As a result, the yield of the liquid-crystal display device is reduced.
To avoid the above problems, a peripheral short-circuiting ring is formed so as to surround a plurality of panel regions on a common glass substrate including the panel regions, and the TFTs within the panel regions are connected to the peripheral short-circuiting ring, thereby preventing the accumulation of electric charges on the substrate.
FIG. 4 shows an example of a common glass substrate 100 having a peripheral short-circuiting ring formed in each panel region. In FIG. 4, the same components as in the foregoing figures are denoted by the same reference numerals.
As shown in FIG. 4, a plurality of panel regions 100A outlined by scribe regions including scribe lines SL indicated by dotted lines in the figure are formed on the common glass substrate 100. In each of the panel regions 100A, a TFT array constituted by the TFTs 11 shown in FIG. 2 is formed. A scanning-side peripheral circuit 13B that operates in cooperation with the TFT array and selects one of the scanning electrodes 13, and a signal-side peripheral circuit 12B that operates in cooperation with the TFT array and selects one of the signal electrodes 12 are further formed in each of the panel regions 100A. The scanning-side peripheral circuit 13B and the signal-side peripheral circuit 12B are equivalent to the peripheral circuit 1PR described with reference to FIG. 2.
In each of the panel regions 100A, a peripheral short-circuiting ring 15S that extends along the boundary of the scribe region is formed in such a manner as to surround the TFT array and the peripheral circuits 12B and 13B, and each signal electrode 12 and each scanning electrode 13 in the TFT array are connected to the peripheral short-circuiting ring 15S. The pad electrodes 12A and 13A (not shown in FIG. 4), which are formed along the outer periphery of the panel regions, are also electrically connected to the peripheral short-circuiting ring 15S via a terminal short bar 13S.
With the above structure, the static electricity generated in the display region escapes to the peripheral short-circuiting ring 15S via the signal electrodes 12, the scanning electrodes 13, and the short bar 13S, thereby preventing electrostatic damage in the elements formed in the display region. The peripheral short-circuiting ring 15S is removed when the common glass substrate is divided into individual display panels by cutting along the scribe regions.
Meanwhile, in the conventional structure shown in FIG. 4, the pixel electrodes 14 for driving liquid cells or the accumulation volumes C disposed in parallel with the pixel electrodes 14 are connected to the peripheral short-circuiting ring 15S via the TFTs 11. If electric charges are generated in the pixel electrode 14 and the accumulation volume C during the production of the liquid panel, electrostatic damage in the TFTs 11 cannot be effectively prevented even with the peripheral short-circuiting ring 15S. Likewise, if static electricity is generated in the TFTs, charging cannot be prevented by the peripheral short-circuiting ring 15S.
In the conventional procedures of producing the conventional liquid-crystal display device, as shown in FIG. 5A, a testing terminal 16 is disposed between each pad electrode 12A or 13A and each corresponding peripheral circuit 12B or 13B, so that various electric tests can be performed on a display panel that is being produced. Since the pad electrodes 12A and 13A are connected to the peripheral short-circuiting ring 15S at this point, a resistance r0 is interposed between each pad electrode 12A or 13A and the peripheral short-circuiting ring 15S.
Conventionally, the resistance r0 has a constant resistance value of 100 kxcexa9, for instance. As shown in FIG. 5B, a clock T1, a positive power source voltage T2, and a negative power source voltage T3 are supplied to each pad electrode 13A, and these signals or power source voltages are then supplied to a CMOS inverter circuit that constitutes the peripheral circuit 13B. As for each pad electrode 12A and each corresponding peripheral circuit 12B, the same structure is employed.
FIG. 5C is an equivalent circuit diagram of a part including one of the pad electrodes 12A, the corresponding one of the peripheral circuits 12B, and the peripheral short-circuiting ring 15S.
As shown in FIG. 5C, the impedance (rin) of the pad electrode 13A, to which the clock T1 is inputted, is much greater than the resistance r0, because the CMOS inverter circuit has great input impedance. On the other hand, the impedance R at a pad electrode 13A that receives the positive power source voltage and a pad electrode 13A that receives the negative power source voltage can be expressed as:
xe2x80x83R=r0xc3x97(R0+rin)/[r0+(r0+rin)]
wherein rin is the internal resistance of the CMOS inverter circuit. In this case, the value of the internal resistance rin is smaller than r0, and the value rin might vary with each of the pad electrodes 13A. This means that the apparent resistance value between the pad electrodes 13A and the peripheral short-circuiting ring 15S varies with each of the pad electrodes 13A. Meanwhile, if the value of the resistance R varies with the pad electrodes 13A, there is a possibility of causing great potential differences between the pad electrode that receives the clock T1, the electrode pad that receives the positive power source voltage, and the pad electrode 13 that receives the negative power source voltage. With such great potential differences, the MOS transistor that constitutes the CMOS inverter circuit might be electrostatically damaged.
A general object of the present invention is to provide a novel liquid-crystal display device and a method of producing the same in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a method of producing a liquid-crystal display device that can solve the problem of electrostatic damage in the TFTs caused due to charges generated inside the TFTs on the glass substrate during the production of an active-matrix liquid-crystal display using a peripheral short-circuiting ring.
Another specific object of the present invention is to provide a method of producing a liquid-crystal display device that can prevent a potential difference from being caused between the TFTs on the glass substrate due to such conditions as the pixel electrode shape, the wiring area, and the circuit structure, during the production of an active-matrix liquid-crystal display using a peripheral short-circuiting ring.
Yet another specific object of the present invention is to provide a liquid-crystal display device that includes a CMOS circuit in which no variation in threshold value of the TFTs is caused by static electricity.
Still another specific object of the present invention is to provide a method of producing a liquid-crystal display device that can restrict the generation of potential differences in the TFT circuits during the production of an active-matrix liquid-crystal display in which the TFT circuits are connected to a peripheral short-circuiting ring via resistances.
The above objects of the present invention are achieved by a method of producing a thin-film transistor on an insulating substrate, which method comprises the steps of:
forming a polysilicon pattern on the insulating substrate, the polysilicon pattern including a first region of a first conductivity, a second region of the first conductivity, a first bridging region that connects the first region and the second region, and a second bridging region that connects the first region and the second region;
forming an insulating film on the insulating substrate in such a manner that the insulting film covers the polysilicon pattern;
forming a gate electrode pattern on the insulating film in such a manner that the gate electrode pattern covers the first bridging region;
forming a wiring pattern on the first region in such a manner that the wiring pattern is in contact with the first region; and
cutting the second bridging region after the step of forming the wiring pattern.
In the above method, the step of forming the polysilicon pattern includes the step of providing a conductivity to the second bridging region.
The above objects of the present invention are also achieved by a thin-film transistor that includes:
an insulating substrate;
a polysilicon pattern formed above the insulating substrate, the polysilicon pattern including a first region of a first conductivity, a second region of the first conductivity, and a channel region that connects the first region and the second region;
a gate insulating film that covers the channel region; and
a gate electrode formed above the channel region.
In this thin-film transistor, the polysilicon pattern has a first extending portion that extends from the first region to a first tip end, and a second extending portion that extends from the second region to a second tip end.
The above objects of the present invention are also achieved by a liquid-crystal display device that comprises:
a first glass substrate;
a second glass substrate that faces the first glass substrate, with a gap being maintained between the first glass substrate and the second glass substrate;
a liquid-crystal layer enclosed in the gap; and
a thin-film transistor formed on a surface of the first glass substrate, the surface facing the second glass substrate.
In this liquid-crystal display device, the thin-film transistor includes:
a polysilicon pattern formed on the surface of the first glass substrate, the polysilicon pattern including a first region of a first conductivity, a second region of the first conductivity, and a channel region that connects the first region and the second region;
a gate insulating film that covers the channel region; and
a gate electrode pattern formed on the channel region.
In this thin-film transistor, the polysilicon pattern has a first extending portion that extends from the first region to a first tip end, and a second extending portion that extends from the second region to a second tip end.
The above objects of the present invention are also achieved by a thin-film transistor substrate that comprises:
a glass substrate having a panel region formed thereon;
a conductive peripheral short-circuiting ring that is formed in the panel region on the glass substrate and extends along the boundary of the panel region without a gap;
an internal circuit that is formed in the panel region on the glass substrate, includes a plurality of thin-film transistors formed on the glass substrate, and is provided with a plurality of connection terminals; and
a plurality of resistance elements that are formed in the panel region on the glass substrate, each of the plurality of resistive elements electrically connecting a respective one of the plurality of connection terminals to the peripheral short-circuiting ring.
In this thin-film transistor substrate, the resistance values are selected such that the impedance at each connection terminal is substantially constant.
With the above method and liquid-crystal display device of the present invention, the problem of uneven potential difference that has been caused by the differences in electrode shape and area can be eliminated by adding a bridging region that bridges the source region and the drain region to a polysilicon pattern that includes the source region, the drain region, and the bridging region of a thin-film transistor. Since the step of cutting the bridging region is performed at the same time as the step of forming a contact hole in the thin-film transistor, the number of production steps can be prevented from increasing. The present invention is particularly effective in the production of an active-matrix liquid-crystal display in which a large number of plasma processes are performed.
With the thin-film transistors of the present invention, the problem of electrostatic damage or deterioration of thin-film transistors that constitute the internal circuit, due to potential differences induced by differences in impedance at the connection terminals, can be eliminated from the internal circuit.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.